1. Field of the Invention
This invention relates generally to systems for measuring magnetostriction in thin magnetic films and more particularly to such a system employing multiple harmonics of an external rotating magnetic field to improve measurement accuracy.
2. Description of the Related Art
Areal data storage densities are increasing at an unprecedented rate owing to advances in the development of magnetic medium and read head materials. Exploiting increasing storage density requires increased signal output from the read sensor. For this reason, thin film inductive read heads were eventually replaced by read heads using the anisotropic magnetoresistance (AMR) effect. Subsequently, a class of metallic multilayer films exhibiting the giant magnetoresistive (GMR) effect were introduced as read heads for hard-disk (HD) and tape magnetic storage devices. GMR effect read sensors exhibit an MR ratio, xcex94R/R, that is typically 8% or higher compared with perhaps 2% for earlier AMR sensors. The increased sensitivity allows acceptable signal levels from smaller read element track widths, thereby increasing track density and areal storage density.
The exchange biased spin-valve (SV) is a multilayer GMR device that is most useful for read head applications. A SV consists of several layers: a free and pinned layer both made of a soft ferromagnetic (FM) material such as Permalloy (NiFe) or a cobalt (Co) alloy. These two FM layers are separated by a non-magnetic conductive spacer layer such as copper (Cu). An exchange layer of antiferromagnetic (AFM) material, commonly made of a manganese (Mn) alloy such as NiMn, is deposited next to the FM pinned layer. The FM free layer is thin enough to allow conduction electrons to frequently move back and forth between the free and pinned layers via the conducting spacer layer. The magnetic moment of the FM pinned layer is fixed and held in place by the AFM layer, while the FM free layer magnetic moment changes in response to the external magnetic field, such as that from a bit stored on a hard disk.
The quantum spin property of electrons, i.e., either spin up or spin down, is exploited in SV sensors. Conduction electrons with spin parallel to the FM material""s magnetization (spin xe2x80x9cupxe2x80x9d) move freely, while the motion of those electrons with anti-parallel orientation (spin xe2x80x9cdownxe2x80x9d) is impeded via collisions with atoms in the material. When the FM free and pinned layer magnetic moments are parallel, spin up electrons move freely in both FM layers, corresponding to a relatively low effective resistance. Conversely, when the free and pinned layer magnetic moments are anti-parallel, movement of spin up electrons is hampered by one layer, while the movement of spin down electrons is hampered by the other, so that neither move freely through both FM layers, leading to a relatively high effective resistance. In the GMR sensor, the external field from a recorded bit rotates the FM free layer magnetic moment relative to that of the FM pinned layer, effectively switching the SV device between the high and low resistance states.
Much of the cost in manufacturing read head sensors is incurred in processing the individual sensors after the actual material deposition process. So a means for qualifying the post deposition product is critical to reduced cost (improved yield). In fabricating GMR SV devices, a variety of in-process metrology is required to provide requisite material deposition process control. In-process measurement of magnetic and magnetoresistive properties is one method by which the film deposition process is qualified and controlled. One exemplary concern is the uniformity and thickness of the deposited layers (the copper spacer layer is typically less than 15 atoms thick). Another is surface roughness, which affects the coupling between layers, the coercivity of the FM free layer, and the effectiveness of the AFM layer in pinning the FM pinned layers. Other concerns include the thin-film properties that significantly affect device performance, such as magnetoresistance, resistivity and magnetostriction. Many of these parameters interact with one another, so fabricating acceptable read sensors requires strict quality control tolerances and processes.
The saturation magnetostriction in the magnetically soft GMR films may induce undesirable magnetic anisotropy changes during head fabrication and therefore must be tightly controlled. Anisotropy is controlled during wafer production using a complex combination of process parameters during magnetic film deposition. Eventually, the wafers are diced and the critical aerodynamic surfaces are polished. Both of these mechanical processes create unpredictable changes in the stress level between the substrate and the magnetic films. The magnetostriction of the film material translates these unpredictable stress changes into changes in the anisotropy of the magnetic layer. Also, an inverse relationship between applied magnetostrictive stress and the MR sensitivity ratio, xcex94R/R, has been noted. The preferred way to control anisotropy after deposition is to keep magnetostriction below 10xe2x88x927. Accordingly, the accurate measurement and analysis of thin-film magnetostriction is critical to effective process control during manufacture of modern GMR SV sensors.
Determination of the magnetostriction coefficient of ferromagnetic materials by inference from the measurement of the deflection in an external magnetic field Hext of a substrate element on which a layer of the material under test has been applied has been known in the art for several decades. As understood in the 1960""s and 1970""s, this procedure suffered from numerous well-known disadvantages, including the difficulties of accurate characterization of substrate material parameters, accurate detection of microscopic deflections, heating effects from the requisite external field intensity, unsuitability of the requisite procedures to automation, and the like.
A major improvement in this technique was disclosed in the commonly-assigned U.S. Pat. No. 4,310,798 issued to Brunsch et al. and entirely incorporated herein by this reference. The Brunsch et al. invention optically measures the dynamic displacement of the free end of a cantilever substrate element on which is applied a layer as thin as 5 nm of the FM material under test, in the presence of an external rotating magnetic field. The substrate element deflection arising from magnetostriction of the test material in the external rotating magnetic field occurs at twice the rotation frequency f so this rotation frequency f is tuned to half of the mechanical resonant frequency f0 of the cantilever element to permit accurate detection of the deflection amplitude, which is otherwise too small. Because Brunsch et al. assume the test material to have a linear magnetization characteristic, leading to a quadratic dependence of magnetostriction xcex on external field intensity Hext, their procedure looks for a maximum resonant deflection RMS amplitude A0 and presumes it to be proportional to the saturated magnetostriction xcexS. The exact proportionality is a function of substrate material characteristics. The Brunsch et al. procedure resolved several problems known in the art and, for the first time, permitted automated magnetostriction testing, but did not eliminate the need for accurate characterization of the substrate material parameters.
Later, Cheng et al. (Cheng et al., xe2x80x9cDevice to Measure Magnetostriction of Thin Film on a Substratexe2x80x9d, IBM Technical Disclosure Bulletin, July 1988, pp. 59-60) propose an improvement of the Brunsch et al. procedure that adds a lock-in amplifier to the deflection amplitude sensor that permits operation well below mechanical resonant frequency f0, thereby distinguishing the magnetostrictive deflection A2 at 2f from many other sources of resonant deflection at f0. The lock-in amplifier provided enhanced sensitivity needed to accurately detect the microscopic deflection amplitudes below mechanical resonance.
The Cheng et al. proposal was later improved by Tam et al. (Tam et al., A New High-Precision Optical Technique to Measure Magnetostriction of a Thin Magnetic Film Deposited on a Substrate, IEEE Trans. Magn., vol. 25, no. 3, pp. 2629-38, May 1989) by adding a piezoelectric actuator for in situ calibration of the substrate element contributions to the measured deflection A2 at 2f for removal from the magnetostrictive deflection measurements. By exciting the detector with a separate piezoelectric actuator at rotation frequency f and measuring the resulting Hext=0 deflections {A2} before and after measuring the deflection A2 in a nonzero rotating external magnetic field intensity Hext, the unwanted effects on the reflected signal of substrate material reflectivity and curvature may be eliminated from the magnetostrictive measurements. As with Cheng et al., all deflection measurements {A2} are made well below mechanical resonant frequency f0 and a lock-in amplifier is used to measure the RMS deflection A2 at the second rotation harmonic frequency 2f, which is presumed to be proportional to the magnetostriction coefficient xcex.
At about the same time, Arai et al. (Arai et al., xe2x80x9cMeasurement of Thin Film""s Magnetostriction with Piezoelectric Ceramic Substrates,xe2x80x9d IEEE Trans. Magn., vol. 25, no. 5, pp. 4201-3, September 1989) made a similar proposal to improve the art by using a piezoelectric substrate and measuring the piezoelectric drive voltage required to exactly cancel the magnetostrictive strain-induced deflection A2 of the free end of a cantilever substrate element at the second rotation harmonic 2f. But accurate knowledge of the substrate piezoelectric constant d31 is necessary to determine the desired magnetostriction coefficient xcex using the Arai et al. technique, which does not eliminate the many other disadvantages of the original cantilever deflection second-harmonic amplitude method.
Later, Bellesis et al. (Bellesis et al., xe2x80x9cMagnetostriction Measurement by Interferometry,xe2x80x9d IEEE Trans. Magn., vol. 29, no. 6, pp. 2989-91, November 1993) introduced Doppler interferometry to the cantilever deflection measurement process, thereby improving deflection measurement precision to about 0.01 nm. Using a smaller sample size, higher rotation frequencies may be used to avoid many common spurious vibrations. Bellesis et al. note that the deflection amplitudes {Am} at rotation harmonics {fm} other than the deflection A2 at the second harmonic f2 may be observed with their technique (assuming that none are close to mechanical resonance f0) and they speculate as to the source and significance of the various harmonics, but neither consider nor suggest using the other rotation frequency harmonics {fm} to implement an improved automated magnetostrictive measurement process.
Similarly, Rengarajan et al. (Rengarajan et al. xe2x80x9cMeasurement of Very Low Magnetostrictions in Thin Films,xe2x80x9d IEEE Trans. Magn., vol. 31, no. 6, pp. 3391-3, November 1995) proposes using a highly-sensitive interferometer with in situ calibration, thereby improving deflection measurement precision by an order of magnitude over Bellesis et al. (to about 1.0 pm). Rengarajan et al. suggest using close-coupled copper strips instead of Helmholtz coils to permit even smaller substrate element sizes and higher rotation frequencies than those proposed by Rengarajan et al., thereby removing even more spurious vibrations from the measurements.
For several years, the commercial instruments available for measuring magnetostriction in thin films have included the LaFouda LAMBDA09 Automated Magnetostriction Tester (available from LaFouda Solutions, San Diego, Calif.), which automates the Tam et al. sub-resonant rotating-field second-harmonic cantilever displacement amplitude technique discussed above. Even in view of the dramatic increases in precision made possible by Bellesis et al. and Rengarajan et al., this technique still suffers from several well-known disadvantages that affect accuracy. The procedure relies on several assumptions that are increasingly problematic. Until the advent of the most recent GMR sensor designs, these accuracy problems were commercially tolerable.
For example, the rotating field frequency is selected to cause the substrate element cantilever to oscillate at twice the field rotation frequency f and the magnetostriction coefficient xcex is derived from the measured second-harmonic deflection amplitude A2. This approach relies on a simple quadratic relationship between magnetostrictive strain and applied magnetic field intensity, which in turn assumes a linear magnetization curve in the material under test. This simple quadratic relationship also assumes that the internal magnetic moment Mint in the test material tracks the external applied field Hext in phase, which it clearly cannot until well above saturation because of the various effects on internal moment Mint from local coupling and magnetic anisotropy. The assumed magnetization curve linearity is valid only for an ideal uniaxial anisotropy sample with the external applied field Hext oriented along the hard axis of the sample. Real materials exhibit non-linear magnetization curves when the external applied field Hext is oriented along any other axis, as is well-known in the art. In the existing rotating field measurement method, the external applied field Hext is applied along all axes, giving many other non-quadratic measured magnetostrain terms which, until now, have been ignored in the art. A favored method for reducing the resulting measurement errors is to drastically increase the external applied field Hext to force Mint well above saturation in the sample, but this is now increasingly problematic in modern SV designs because of increased noise from eddy current effects and increased laser signal distortion from the thermal convection air turbulence caused by the heating effects of the high field coil currents required.
These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.
This invention solves the accuracy problems by for the first time including the effects of the harmonic cantilever deflection amplitudes {Am} other than the deflection amplitude A2 measured at the second rotation harmonic frequency f2. For example, this may be accomplished by adding two additional lock-in amplifiers for detecting the respective deflection amplitudes {A1, A3} at the first and third harmonics {f, f3}. Using an external field intensity Hext equal to the sample saturation moment Msat and assuming a 15 Oe sample anisotropy Hk and coupling bias Hp, the saturation magnetostriction coefficient xcexS measurement error may be reduced by 90% over the error from using only the second harmonic deflection amplitudes A2. Even when the external field intensity Hext is boosted to twice the sample saturation value in attempting to overcome these anisotropy Hk and coupling bias Hp effects, the method of this invention may reduce measurement errors by 80%. Additional harmonic cantilever deflection amplitudes {Am} may provide additional accuracy in the measured saturation magnetostriction coefficient xcexS.
It is a purpose of this invention to improve accuracy of magnetostriction measurements at a predetermined external field intensity Hext by incorporating deflection measurements at additional rotation harmonic frequencies to correct for the effects of magnetic moment phase distortion in the material under test while retaining the advantages of lock-in amplifier signal detection.
In one aspect, the invention is a machine-implemented method for measuring the magnetostriction coefficient xcex of a sample material applied to a substrate element having two ends, one end being fixed and the other end free to be deflected, including the steps of (a) exposing the substrate element to an external rotating magnetic field having an intensity Hext and a rotation frequency f, (b) detecting the amplitude Am of the deflection of the free end at each of a plurality of frequencies {fm} each representing a harmonic of the magnetic field rotation frequency f, and (c) combining signals representing a plurality of the harmonic deflection amplitudes {Am} to determine the magnetostriction coefficient xcex of the sample material.
In a preferred embodiment, the invention is an apparatus for measuring the magnetostriction coefficient of a sample material applied to a substrate element having two ends, including means for fixing the substrate element at one end leaving the other end free to be deflected, means for creating an external rotating magnetic field having an intensity Hext at the fixed substrate element and a rotation frequency f, means for measuring the amplitude Am of the deflection of the free end at each of a plurality of frequencies {fm} each representing a harmonic of the rotation frequency f, and processor means for combining signals representing a plurality of the harmonic deflection amplitudes {Am} to determine the magnetostriction coefficient xcex of the sample material.